Somehow I just haven't felt like implementing them.
Maybe it's just I don't want to make a full ZIP release every time I change something, especially if it's a tiny change.
Another thing is that I hardly even *use* the editor, so I don't have any interest in improving it.
Maybe I test a few things now and then, but I haven't made any complete levels, let alone mods with it.

I have moved my other big project, SDLPoP to GitHub. (And also PR, although I had less part in it than in SDLPoP.)
I'd like to do the same with this editor.

But to fully take advantage of GitHub, I also want to make it possible for everyone to compile the editor for themselves.
For that, I will have to abandon C++Builder and move to something free and cross-platform.
(FYI, I don't even have C++Builder installed at the moment.)

Lazarus (http://lazarus-ide.org/) seems promising for this purpose.
It's free: The IDE is licensed under the GPL, and the libraries (GUI, etc.) are licensed under the LGPL.
Another important thing is that its libraries are interface-compatible with Delphi, and therefore with C++Builder.
So it shouldn't be too hard to rewrite the editor for the new libraries.
I have to convert C to Pascal, though. With that, I'm not sure if the compatibility is still such a big advantage.

Above all, I'd say work on something that you feel motivated to work on.
Related: the editor is mostly feature-complete, so would other GitHub users feel motivated to work on it even if you'd move away from C++Builder?
Also, is it worth the trouble, given that only ~2.5 SNES mods are being created each year, probably all on Windows?

If you decide the invest the time and energy, then my advice would be to stick with C(++) and use Qt Creator. Related post here.
Qt Creator and Qt are widely used (VirtualBox, OBS, Skype, VLC, etc.), have good documentation and a helpful community.
You wouldn't have to rewrite the editor from scratch, you'd have to split the UI/visual code from the logic/core code.
(Most of the heavy code is related to loading/saving imagery and such, right?)
Maybe you could start with the latter and look for a volunteer in the Qt community?

Above all, I'd say work on something that you feel motivated to work on.
Related: the editor is mostly feature-complete, so would other GitHub users feel motivated to work on it even if you'd move away from C++Builder?
Also, is it worth the trouble, given that only ~2.5 SNES mods are being created each year, probably all on Windows?